The invention relates to a method of implementing macrodiversity in a cellular radio network having in each cell at least one base station which communicates with subscriber terminals within its area, and in which method packet switched connections are provided between the base station and the terminals, the connections comprising an actual traffic channel and a separate control channel, and that in signal transmission, the terminals use IQ modulation in which the traffic and control channels are multiplexed to be transmitted on different branches, and the terminals can communicate with more than one base station simultaneously.
The present invention is suitable for use particularly in a cellular radio system utilizing code division multiple access. CDMA is a multiple access method based on the spread spectrum technique, and it has recently been applied to cellular radio systems, in addition to the prior FDMA and TDMA methods.
In a typical mobile telephone environment, signals between a base station and a mobile station propagate between a transmitter and a receiver over several paths. This multipath propagation is mainly caused by the reflections of the signal from the surrounding surfaces. The signals having propagated over different paths reach the receiver at different times because of a different propagation delay. In CDMA, the multipath propagation can be utilized in signal reception in the same manner as diversity. The autocorrelation properties of the spreading codes used in transmissions enable the separation of different delay components from one another. A widely used CDMA receiver solution is a multi-branch rake receiver structure in which each different branch is synchronized with a signal component having propagated over a different path. A digital receiver unit is composed of a number of rake branches, and each branch is an independent receiver element whose task is, in other words, to despread and demodulate one received signal component. The CDMA receiver advantageously combines the signals of different digital receiver unit elements, whereupon a signal of good quality is obtained.
In CDMA systems, it is also possible to apply soft handover in which the mobile station can communicate simultaneously with several base stations. This is also referred to as macrodiversity. Consequently, the connection quality of the mobile station during handover remains high, and the user does not notice a break in the connection. In the conventional macrodiversity, two or more base stations transmit the same signal in the downlink transmission direction (from base station to terminal). Since the base stations use the same frequency, the terminal may receive simultaneously from more than one transmitter. The signals from different base stations are separated in the same manner as the delay components by means of different rake branches. In the uplink transmission direction (from terminal to base station), two or more base stations receive the same signal transmitted by the terminal. The signals are combined at a first common point on a signal path. The macrodiversity enables optimal power control, which minimizes the interference level of the network and, consequently, maximizes the network capacity.
Circuit switching is a method in which a connection is set up between users by allocating the connection a predetermined amount of transmission capacity. The transmission capacity is allocated solely to said connection for the duration of the entire connection. Known mobile telephone systems, for example the GSM-based GSM 900/DCS 1800/PCS 1900 systems and the United States IS 95 radio system using the CDMA technique, are therefore based on circuit switching.
Packet switching is a method in which a connection is set up between users by transmitting data in the form of packets that include address and control information, in addition to actual information. Several connections can use the same data link simultaneously. An ARQ protocol is often used in connection with packet protocols. The ARQ protocol (Automatic Repeat Request) refers to a procedure which attempts to improve the reliability of the data to be transmitted by retransmitting the information that is transmitted. In accordance with the protocol, the receiver transmits a data repeat request to the sender if the receiver considers the received data unreliable. The unreliability of the data is detected by checking a check sum from the received packet, for example.
The use of packet switched radio systems particularly for data transmission has been a subject of research in recent years, since the packet switching method is well suitable for data transmission where data to be transmitted is generated in bursts, required by the use of interactive software, for example. In that case, it is not necessary to reserve a data link for the duration of the entire connection, but only for the duration of packet transmission. This provides a considerable saving on cost and capacity during both the construction stage and the operating stage of the network.
When packet switched connections are applied to the CDMA systems, implementing the macrodiversity has been problematic. In the packet switched connections, traffic is not continuous but occurs in bursts. In the conventional macrodiversity, particularly in the transmission direction from the base station to the terminal, the transmission from several base stations generates much interference and it is difficult to control the retransmission of the packets between several base stations. Consequently, the packet switched connections are implemented in prior art solutions without macrodiversity. Since the macrodiversity is not used, all handovers must be implemented as so-called hard handovers in which the connection to the old base station is broken before a connection is set up to a new base station. This has brought about problems particularly in the selection of the new base station and in power control particularly at the borders of the coverage areas.
An object of the invention is to provide a method to solve the above-described problems. This is achieved by a method of the type presented in the introduction, which is characterized by maintaining the actual traffic channel connection between the terminal and only one base station, and by maintaining a control channel connection simultaneously between the terminal and more than one base station.
The method of the invention provides many advantages. The invention enables optimal power control and ensures that the packet traffic can always be allocated the best connection at a given time. Furthermore, interference caused by the packet traffic is reduced in the system, since less traffic occurs in the transmission direction from the base station to the terminals. The automatic repeat request ARQ protocol can also be explicitly implemented. Furthermore, handover to the packet channel connection can be implemented rapidly and reliably, since the control channel connection provides links to the base stations.